The present invention generally relates to AC switching devices and, more particularly, to an AC switching device of the type which is suitable for switching an AC voltage boosted by a transformer in response to on-off commands delivered thereto.
In a facsimile transceiver for example, a recording sheet undergone development is transferred to a fixing station whereat it is illuminated by flashing light successively by each predetermined width. For this flash fixation, it is necessary to boost a commercial AC voltage by means of a transformer, charge a capacitor with the boosted voltage through a rectifier and then supply the charge from the capacitor to a flash lamp at a predetermined proper timing. This in turn requires that the AC voltage applied to the capacitor or like load be turned on and off at predetermined timings.
For such a supply of a voltage boosted by a transformer for each given period of time at a given timing, it has been a common practice to furnish the secondary winding or side of the transformer with suitable switching means such as a Triac and turn it on and off based on on-off commands at zero-crossing points of the alternating current. A drawback inherent in this type of expedient is that it needs a switch which is resistant to a high voltage and, therefore, costly.
The need of such a costly switch resistant to a high voltage will be precluded if an arrangement is so made as to turn on and off the primary winding side of the transformer instead of the secondary side. However, should the primary side of the transformer be turned on and off at improper timings, the iron core of the transformer would become saturated resulting in the flow of a very large rush or inrush current into the primary side of the transformer. More specifically, if the primary side of the transformer is turned off at a zero-crossing point where the alternating current traverses the zero level from the positive phase to the negative and then turned on at a zero-crossing point where the alternating current traverses the zero level from the negative phase to the positive, the iron core of the transformer will become saturated to allow a rush current of a magnitude which is about 20 times the usual current to flow into the primary side of the transformer. This inflow of a rush current into the primary side cannot be avoided unless the transformer is provided with an iron core which is large enough to prevent the transformer from being saturated even if the on-off timings of the primary side were disturbed. The result is an increase in both of the cost and overall dimensions of AC switching device.